96 Electrical Potentials of the Brain /5 : 4 



the control of voluntary muscular action. When a person wills to write 

 his name with a pen or to scratch his back, many negative feedback loops 

 are active. The person does not consider how to move each muscle fiber 

 or how to tense one muscle and relax its opponent. Rather, his nervous 

 system, at a level below consciousness, finds the deviation from the 

 desired location and controls the muscles to reduce this deviation. If the 

 action can be seen, feedback loops involving the visual system are the 

 most important (to humans). There are, in addition, sense organelles 

 within every muscle called proprioceptors, which send signals to the 

 central nervous system "describing" the tension in that muscle. The 

 feedback loops involving proprioceptors are important for back-scratch- 

 ing, walking, and many other activities in which one does not see the 

 limb which is controlled. 



The presence of many coordinated negative feedback loops in the 

 nervous system is analogous to a large electronic or mechanical computer. 

 Moreover, the fact that neurons obey an all-or-none principle makes 

 them correspond to the elements of an electronic digital computer. An 

 additional feature common to many of the giant electronic "brains" is 

 a scanning device which sweeps continuously over the elements of the 

 "memory." If this type of phenomenon occurs in the brain, it seems 

 reasonable to look for it in the integrated potentials at the surface of the 

 cerebral cortex. These potentials are called eegs; the remainder of the 

 chapter is devoted to their description, analysis, and applications. 



4. The Electroencephalographic Patterns 



Changing electrical potentials are always found on the surface of the 

 brain of a living vertebrate ; that is, an eeg pattern can always be found. 

 This indicates that the nerve fibers in the central nervous system are 

 never at rest. By way of contrast, the nerves to a relaxed or anesthetized 

 limb may be completely inactive for long periods. The continual 

 activity of the central nervous system might indicate some type of 

 scanning system or, perhaps, the storage of memory in neural feedback 

 loops. However, neither of these is supported by direct experimental 

 evidence. 



The potentials in an adult human between the surface of the cortex 

 and a neutral region, such as an ear lobe, are as large as 10 mv. They 

 change more slowly than the sharp spike potentials of individual neurons. 

 The eeg potentials represent an average of the changes in a large number 

 of neurons. By and large, the potentials travel over the surface of the 

 cortex, so that, for a distance of a few millimeters, there is a time differ- 

 ence in the eeg pattern rather than a major difference in form. 



